In order to maintain machines and equipments at their optimum performance, the precautionary servicing and maintenance engineering has gained its importance and is currently effectively introduced in the factory workshop.
It has long been well known that, while most of the machines and equipments employed in various fields of industries utilize a rotating and a reciprocating machine, the presence of an abnormality in any one of the rotating and reciprocating machines can be determined when it is found that the overall amplitude of vibration detected from the machine becomes abnormally higher than the amplitude of vibration during a normal operating condition of the machine. In other words, the presence and absence of an abnormality in the machine can be determined by the magnitude of the overall amplitude of vibrations generated by such machine.
So far as the rotating machine is concerned, it is also well known, as it is actually performed, that the location of a trouble and a cause therefor can be specifically determined by detecting and analyzing the frequency spectra of vibrations and then by calculating the individual amplitudes of the frequency spectra.
According to the prior art, the trouble-shooting of the rotating machine is carried out by bringing a vibration recorder to the site of installation of the machine to be examined, recording data of vibrations of the machine on a magnetic recording tape, and analyzing the recorded vibration data by the use of a large-sized spectrum analyzer at a research laboratory or a similar establishment remote from the site of installation of the machine. This technique is far from the real-time analysis and is, therefore, inconvenient and timeconsuming.
In view of the above, there has been proposed a diagnosing apparatus incorporating therein a frequency spectrum analyzer utilizing a digital-type fast Fourier transform (FFT). With this diagnosing apparatus, the real-time determination of the presence and absence of an abnormality in the rotating machine can be performed in situ.
Specifically, this diagnosing apparatus weighs 10 to 30 Kg and for use is, brought to the site of installation of the rotating machine for the real time frequency analysis carried out in such a manner that, when a significant amplitude spectrum appears in a particular frequency component of the fundamental frequency which is a function of the number of revolution of the rotating machine being then examined, the particular frequency component containing the significant amplitude spectrum is checked against the frequencies of vibrations which have been generated by such machine and which have previously been theoretically classified according to the location of a trouble and the cause therefor, thereby to find the location of a trouble and the cause therefor.
As is well known to those skilled in the art, the frequency of vibrations generated by a rotating machine during occurrence of an abnormality, the location of the abnormality in such machine and the cause of such abnormality can be theoretically determined and have such a correlation as tabulated in FIG. 1 of the accompanying drawings.
Although the abnormality diagnosing apparatus incorporating the frequency spectrum analyzer using the fast Fourier transform (FFT) has the advantage that the determination of the presence and absence of the abnormality can be performed in situ on the real-time basis, it has the disadvantage that calculation of the distribution of frequency spectra according to the FFT system gives a limited resolving power because of the calculation being performed on a digital system. That is to say, this resolving power is determined by the number of bits of a register used for data calculation so far as the amplitude of the calculated frequency spectrum is involved. By way of example, if the number of bits of the register is eight, the resolving power will be limited to 1/256 of the full-scale input whereas, if it is ten, it will be 1/1024 of the full-scale input.
Still further when the frequency spectra is calculated by means of the FFT system, the frequency resolving power takes a limited value. As is well known, the frequency resolving power is determined by the FFT system according to the sampling time (time interval required to complete analog-to-digital conversion) and the number of input data.
By way of example, assuming that the sampling time interval is expressed by .DELTA.t, and the number N of the input data is expressed by 2.sup.m, the maximum value f.sub.M of the frequency analyzed is 1/2.DELTA.t, and the resolving power .DELTA.f of the frequency analyzed is expressed by the following equation. EQU .DELTA.f=2f.sub.M /N=2f.sub.M /2.sup.m
In other words, the frequency spectra are analyzed to give results at intervals of .DELTA.f up until the maximum, frequency f.sub.M is analyzed. Therefore, the number of the frequency spectra is N/2.
Where an input representative of the frequency of vibrations detected is a function of time relative to a frequency f.sub.m other than the frequency expressed by n.DELTA.f (wherein n is an integer, 0, 1, 2 . . . N/2-1), that is, where f(t) =A sin 2.pi.f.sub.m t, the result of spectrum analysis subjected to this input by means of the FFT system does not show a linear spectrum of a single frequency, but a mixture of various frequency spectra.
By way of example, assuming that the number of the input data is 1024 points and the sampling time interval is 9.765.times.10.sup.-4, the maximum value of the analyzed frequency is expressed as follows. EQU f.sub.M =1/2.DELTA.t=512 Hz EQU .DELTA.t=2f.sub.M /N=1 Hz
and, accordingly, the analyzed frequency range of 1 Hz to 512 Hz can be displayed at intervals of 1 Hz.
Where the input representative of the frequency of vibrations detected has the relationship, f(t) =A sin 2.pi.f.sub.m t, the spectrum will be a linear spectrum if f.sub.m is 50.0H, but it will not be a linear spectrum if f.sub.m is 49.7 Hz, 49.8 Hz, 49.9 Hz, 50.1 Hz, 50.2 Hz . . . or 50.6 Hz.
By way of example, if f.sub.m =49.5 Hz, the frequency spectrum of the rotating machinery is, as shown in FIG. 2 of the accompanying drawing, distributed on both sides of 50 Hz and 51 Hz and is displayed as if spectrum analysis were subjected to an input representative of combined harmonic vibrations, not of a single harmonic vibration.
This means that, when a vibration of a cycle other than an integer multiple of the resolving power of the analyzed frequency is generated, an accurate spectrum analysis can not be achieved.
Because of the reason discussed above, even when the diagnosing apparatus operating on the basis of the spectrum analysis is used in situ for the real-time diagnosis of the abnormality in the rotating machinery, the absolute value of the vibration amplitude can not be accurately measured where the frequency of vibrations produced is not an integer multiple of the unit of frequency representative of the resolving power. Accordingly, not only is the determination of the location of a trouble and the cause therefore not performed accurately, but also a difficulty is involved in the performance of a precautionary measure at the site of installation of the rotating machinery on a real-time basis by way of checking currently available diagnosis data against the previously obtained diagnosis data.